Housing
What size is your tank? 8 gal
What temperature is your tank? 79F/26C
Does your tank have a filter? Yes, baffled
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Aqueon Betta Food pellets
How often do you feed your betta fish? Once a day, only as much as he can eat
Maintenance
How often do you perform a water change? Normally once a week, now more often due to ammonia problems.
What percentage of the water do you change when you perform a water change? 20%
What type of additives do you add to the water when you perform a water change? Stress Coat (Claims to dechlorinate), Tetra FloraPride

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Ammonia: .25
Bringing a cup to LFS to get it tested later today.

Symptoms and Treatment
How has your betta fish's appearance changed? Tail has split; noticed small pinholes develop into larger ones and now it's split all the way down.
How has your betta fish's behavior changed? He's sitting at the bottom and every so often darting around erratically, brushing the sides of his tank; he returns to the spot once done darting.
When did you start noticing the symptoms? This morning. Noticed tail splitting earlier.
Have you started treating your fish? If so, how? 10% water change. Planning on doing this every day.
Does your fish have any history of being ill? No.
How old is your fish (approximately)? Owned for 6 1/2 months, so 1 1/2 years old?

Tail has split; noticed small pinholes develop into larger ones and now it's split all the way down.

Ammonia exposure eats away at the fins. Then fin rot sets in - that's why pinholes and ragged edges can be the first signs of fin rot.

It's really important at this point to make sure he doesn't get any more exposure to ammonia. I'd dramatically increase the amount of water changed, and do so more regularly - or even move him to a hospital tank for a couple of weeks, to make sure the water's absolutely clean. Also, be careful about uneaten food - flakes are really good at producing tons of extra waste, I won't use them at all anymore - I see you use pellets though - does your fish eat every one?

His fins may heal just with super clean water, if the issue hasn't gone too far already. If you do put him in a hospital tank, add a small pinch of AQ salt just to start with (ie, don't add more on the next water change)- if he has nitrite or ammonia issues, it'll help him detoxify a bit. Hopefully he won't need antibiotics to clear up a secondary infection, but keep an eye on him for further signs of rot/illness.

Quote:

He's sitting at the bottom and every so often darting around erratically, brushing the sides of his tank; he returns to the spot once done darting.

First thing - do the flashlight check for velvet. I am not sure how a fish would suddenly get that particular parasite after being alone for 6 months or w/e, but I'd do it anyway just to be able to count it out as a cause.

My guess is, however, that this is nitrites or ammonia irritating him. The answer is: increasing water changes, or a hospital tank until the home tank is sorted out. Maybe the tank is trying to cycle? Your LFS will be able to tell you what's up with the water. Take a sample of your tap water, too, in case there's ammonia in that..

I would recommend Prime as a water conditioner. It knocks ammonia on the head and can also be used in an emergency to de-toxify the tank if there's a sudden spike.

I use Stress Guard with new fish/sick fish/major water changes/stressful situations only - I figure, it might do less to reduce stress if the fish is in it all the time. Also, I trust Prime to take care of my water. It's pretty awesome. Up to you, but try to grab a bottle at the LFS if you can and give it a try.

Definitely an ammonia problem. Tested it myself, came out to .5. Tested it at the store, came out to - wait for it - 1. Eeeek! How the results were so different, I don't know; we were both using API liquid tests. Mine's a brand new kit, and I trust his is, too. Ah well. Point is, had an ammonia problem.

Did three 25% water changes over the next three days and added some quick cycle stuff - SeaChem Stability. I've always heard these things were a scam, so I was skeptical, but the store owner told me to put back the $15 AmQuel and get the $5 Stability instead because he's such a huge fan of it. Okay, sure, you just told me to put the expensive up so I'm pretty sure you're not trying to rip me off.

So after water changes and dosing with Stability, I'm pleased to present to you... this picture. Yellower than mustard. Hoorah!

After just one water change he was acting more normally. Now he's patrolling his tank and sashaying for me like always. Yay. His tail is starting to heal up a bit, but I have a feeling that it will be a while until it is completely repaired. Might add some salt in, might not. Will it hurt my plants?

Thanks for your help. I'll be monitoring ammonia daily until I see it's settled into a good, stable 0, then maybe back off to once a week.

As far as your other questions go... I feed him pellets one by one. I drop one in, he eats it, I drop another in. Very rarely do any escape. I'm starting to feed him freeze dried blood worms, too.

Will definitely look into upgrading to Seachem products. LFS owner said that's all he ever uses on the store tanks and his personal tanks.

Yes Seachem products are fantastic, glad to hear things are looking better. If the ammonia climbs again, you need to do a much larger water change - over 50% at least to physically remove the ammonia instead of diluting it.